ETIENNE MULLER'S BOAT BUILDING
Click here for thoughts on choosing a design
Skegs in sea-kayaks... Should I install one? In my opinion... YES, unless you are building a surf-ski style hull, in which case a rudder would be more appropriate.
Installing a skeg is a slightly complex task, requiring extra time, as well as the cutting of a slot in your precious hull. First time builders can be put off by the thought of the extra stress of installing a skeg, but if one thing is certain, it is easier to do it during the construction than as a retro-fit, and I guarantee, unless your design is extremely directionally stable, you will regret it if you don't.
What kind of skeg? There are some simple effective designs available for DIY, as well as manufactured options that can be fitted into your hull, with either rope and bungie systems for deployment up and down, or with cable inside a plastic sleeve, like the brakes on a bicycle, only heavier.
You can skip the blue bit if you are lazy....
There are many instances where foil aspect, shape, and section are crucial. High aspect foils, in particular, are extremely efficient at generating lift if they are well designed.
High aspect foils, though, are sensitive to changes of angles of attack, and to changes in speed, and a combination of the two, depending on their ratio, section, and point of maximum thickness.
A foil moving through the relatively dense medium of water is prone to physics, even if it is quite small, and moving at relatively slow speeds.
The windsurfing boom of a few decades ago generated a lot of interest, experimentation, and refinement of foils, and improved our understanding of the forces involved in their dynamic use, and not just in modelling and theory.
The lateral forces on a windsurfer skeg, traveling upwind at speed, with the sailor's feet positioned virtually on top of it, his full weight, combined with the forces of the sheeted-in rig pushing sideways against this small surface, are massive, and yet, on the whole, the flow will manage to stay attached, even to a fairly high aspect foil, if it is shaped optimally.
The forces on skegs, when it involves kayaking, are fairly low by comparison, which is just as well, because many kayak skegs are flat shapeless utilitarian slices that have very little hydrodynamic refinement at all ... mine, to some degree, included.
I would think that the rudder beneath a surf-ski would definitely benefit from an efficient foil shape, with a fairly wide wide point that is designed to allow efficient flow at fairly low speeds, up to say 15mph. This is because the rudder is an active part of the boat. It is constantly turning, adjusting, to compensate for correction of travel, and at times may be turned almost perpendicular to the direction of travel, and will be able to take on a more radical angle before stalling if it has an optimised shape.
A kayak skeg is used in a very different way. A skeg, unlike a rudder, is not intended to aid turning, but to inhibit it.
Essentially, one drops the skeg in differing degrees, to reduce the tendency for the stern to slide sideways in beam and following seas.
In beam seas, where speed is not really an issue, any shape will do, as long as the amount of area exposed is adjustable. I would think that, at these speeds, effects of lift, or drag, from such a small surface are going to be impossible to detect or to quantify.
In fast quartering and off-wind conditions shape may become more of an issue, but even in these conditions the forces on a kayak skeg are fairly minimal by contrast to those on say a windsurfer skeg.
In use, I feel no discernible difference in the use of my explorer, with its very basic rattle around flat sectioned pie slice of a skeg, and my rather more refined home made foils or, indeed, the very refined ONNO skeg I have in my NorthStar. They all just do the job for which they are intended. I have sometimes wished I had a skeg, but I have never found myself thinking, "Right now, I wish I had a different skeg."
In the final analysis, here are my bullet points:
Install a skeg, even if you are feeling lazy or uncertain of your woodworking abilities.
The fore/aft location of the skeg probably has more effect on the feel of the boat than any other single refinement.
User friendly convenience in deployment and adjustment must play an important part in the incorporation of a skeg ... especially for the benefit of beginners, who are those most likely to benefit from basic skeg use.
The control should be easy to locate, without reaching or having to look for it...
The skeg should be quick and easy to deploy, adjust, and retract... In beam winds, and off-wind, in conditions where a skeg is going to be helpful, the less fuss required to use the skeg the better ... less time with a hand off the paddle, less loss of control, and less chance of a capsize.
A nervous paddler is going to be less inclined to want to experiment with, or use, a system that is not quick, positive, and easy. Part of the reason I like the good old basic cable system.
A skeg that fits perfectly in its box will jam up with sand. I have found that a mm of wiggle room is enough for the fine beach sand we have around here to wash out, while, at the same time, inhibit the ingress of pebbles. Different locations may require different variations though.
Final thought.... Pie slices are simple to design and fit, practical in use, and possibly less prone to damage in the event of a roll up the beach... Foils are not that much more complex, easy to accommodate in a low profile casing ... but mainly, and most importantly, when showing off to gawkers on the beach, are much cooler.
below are a few links.
Et's skeg design and installation can be found here CLICK.
Nick Schade's skeg plans
Rob Mack's skeg plans
A manufactured retro-fit skeg